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"Professional Corporation" under the Texas law is defined as a corporation organized for the sole and specific purpose of rendering a professional service. - http://www.sos.state.tx.us/corp/index.shtml
Professional corporations doing business in Texas are subject to franchise tax.
If the Professional Corporation elected to be treated as S-corporation - you will use form 1120S for S-corporation:
Each shareholder had to calculate taxes based on his/her personal situation. S-corporation would issue Schedule K-1 - http://www.irs.gov/pub/irs-pdf/f1120ssk.pdf for distribution that would be added to the taxable income. Each shareholder will include that amount in personal 1040 and tax will be calculated individually based on total income and deductions.
Please let me know if you need any clarification.